Question: Suppose the diameter of a circle is ${4}$. What is its area?
Answer: ${d = 4}$ ${K = 4\pi}$ First, find the radius: $r = d/2 = {4}/2 = {2}$ Now find the area: $K = \pi r^2$, so $K = \pi \cdot {2}^2 = {4\pi}$.